1. Field of the Invention
The present invention relates to a head used for thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with near-field light (NF-light), thereby anisotropic magnetic field of the medium is lowered, thus data can be written. The present invention especially relates to a thermally-assisted magnetic recording head provided with an element that converts light received from a waveguide into NF-light. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
With the explosion in the use of the Internet in these years, a huge amount of data that are incommensurably larger than ever are stored and used on computers such as servers and information processing terminals. This trend is expected to further grow at an accelerated rate. Under these circumstances, demand for magnetic recording apparatuses such as magnetic disk apparatuses as mass storage is growing, and the demand for higher recording densities of the magnetic recording apparatuses is also escalating.
In the magnetic recording technology, it is necessary for magnetic heads to write smaller recording bits on magnetic recording media in order to achieve higher recording densities. In order to stably form smaller recording bits, perpendicular magnetic recording technology has been commercially implemented in which components of magnetization perpendicular to the surface of a medium are used as recording bits. In addition, thermally-assisted magnetic recording technology that enables the use of magnetic recording media having higher thermal stability of magnetization is being actively developed.
In the thermally-assisted magnetic recording technology, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium where data is to be written; just after that, writing is performed by applying write magnetic field (write field) to the heated portion. Generally proposed is a method in which the magnetic recording medium is irradiated and heated with near-field light (NF-light). The spot of the NF-light is set to be minute; the very small spot size can be realized which is free of diffraction limit. For example, U.S. Pat. No. 6,768,556 and U.S. Pat. No. 6,649,894 disclose a technique in which NF-light is generated by irradiating a metal scatterer with light and by matching the frequency of the light with the resonant frequency of plasmon excited in the metal.
As described above, various kinds of thermally-assisted magnetic recording systems with elements that generate NF-light have been proposed. Meanwhile, the present inventors have devised a technique in which laser light is coupled with a surface plasmon generator in a surface plasmon mode and excited surface plasmon is propagated to an opposed-to-medium surface, thereby providing NF-light, instead of directly applying the laser light to an element that generates NF-light. In the surface plasmon generator, its temperature does not excessively rise because light (waveguide light) that propagates through a waveguide is not directly applied to the surface plasmon generator. As a result, there can be avoided a situation in which the end of a read head element, which reaches the opposed-to-medium surface, becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the generator, which makes it difficult to properly read servo signals during recording operations. In addition, there can also be avoided a situation in which the light use efficiency of an optical system for generating NF-light including the waveguide and the generator is degraded because thermal fluctuation of free electrons increases in the generator. Here, the light use efficiency is given by IOUT/IIN(×100), where IIN is the intensity of laser light incident to the waveguide, and IOUT is the intensity of NF-light emitted from a NF-light generating end of the generator.
A challenge for thermally-assisted magnetic recording using NF-light is to further reduce the size of the spot of NF-light on a magnetic recording medium irradiated with the NF-light. In real thermal-dominant type thermally-assisted magnetic recording, the spot size of the NF-light determines the size of a record bit on a magnetic recording medium. Accordingly, in order to achieve higher recording densities, the spot size needs to be further reduced.
In the surface plasmon generator described above, one way to reduce the NF-light spot size is to reduce the apex angle and curvature radius of an edge of the surface plasmon generator. The reduction reduces the electric field distribution of the NF-light generated from the surface plasmon generator. As a result, the spot size of the NF-light required for thermal assist can be reduced. However, the reduction of the apex angle and the curvature radius also decreases the volume of the whole surface plasmon generator, which leads to the problem of temperature rise described above in the surface plasmon generator as well, although not as serious as in a metal scatterer. Therefore, there is a need to develop another effective means to improve the light density of NF-light to achieve higher recording densities.